The present invention relates to crosslinked solid polymer electrolytes for use both in a small capacity lithium polymer secondary battery, which is used in video cameras, portable information terminals such as cellular phones, notebook computers and the like, and in a large capacity lithium polymer secondary battery, which is used in electric cars, power storage devices for power leveling and the like.
The lithium polymer secondary battery is attracting attention for its importance as one of three core components of the future information industries, the battery being compared to the heart, the semiconductor being compared to the brain, and the LCD device being compared to the eyes. The reason is that the high performance of the battery as an energy source is a necessity in view of the fact that the future electronic apparatuses that will form a necessary part of human life in the 21st century will become portable, of high performance, of light weighted and compact. At present, in small sized information apparatuses such as portable phones, notebook computers and the like, the use of the lithium ion type secondary battery has speedily advanced. Further, the lithium polymer secondary battery is attracting attention as the battery which makes it possible to save energy through load leveling by storing superfluous power, and as the battery for future portable phones and electric cars. Therefore its importance is widely recognized.
Particularly, in the lithium polymer battery for load leveling and for electric cars, a solid polymer electrolyte is preferably used as the ion conduction medium between the anode and the cathode without using a volatile organic solvent. The advantage of avoiding the use of a volatile organic solvent is apparent for the following reason. That is, the secondary battery for such a use is likely to be operated at a temperature of 60xc2x0 C. or higher, and therefore, safety is ensured if a volatile organic solvent is not used.
The solid polymer electrolyte was discovered by P. V. Wright in 1975 (British Polymer Journal, vol. 7, p. 319), and M. Armand named it xe2x80x9cion conduction polymerxe2x80x9d in 1978. Since that time, its use in the electrochemical devices has been gradually increasing. The typical solid polymer electrolyte is composed of polymers having electron-donating atoms such as oxygen, nitrogen and phosphorus, together with a complex of a lithium salt. The most typical example consists of polyethylene oxide (PEO) and a complex of a lithium salt. This shows a low ion conductivity of about 10xe2x88x928 S/cm at room temperature, and therefore, it cannot be applied to the electrochemical devices operating at room temperature. However, it can be used as a power source for electrochemical devices operating at a high temperature.
The ion conductivity of the solid polymer electrolyte is usually increased as the segmental motion of the polymer chain is increased. Therefore, the crystalline region within the polymer structure has to be minimized to increase the non-crystalline region. In connection with this, Blonsky et al. reported on the application of poly(bis(methoxyethoxyethoxy)phosphazine) (J. Am. Chem. Soc., 106, 6945 (1984)). Pantaloni et al. reported on the applicability of poly(ethoxyethoxyethoxy)vinylether (Electrochim. Acta, 34, 635 (1989)). These solid polymer electrolytes which were composed of the above mentioned polymers and a complex of a lithium salt showed an ion conductivity of about 10xe2x88x925 S/cm.
However, the linear or branched solid polymer electrolytes showed poor mechanical properties if they were formed into films. Accordingly, in order to solve this problem, a method of crosslinking the linear or branched polymer chains was introduced. Thus, through the crosslinking, the mechanical properties and the thermal properties were improved. Further, the interaction between the PEO chains was decreased, thereby minimizing the crystallization to improve the ion conductivity.
Crosslinking agents which have been used so far include poly(ethylene glycol) diacrylate and poly(ethylene glycol) dimethacrylate, in which acrylates are bonded to both ends of poly(ethylene oxide) (Japanese Patent Laid-open Nos. Hei-3-56803 and Hei-5-36305, and U.S. Pat. No. 5,571,392). Further, Yuasa Company of Japan obtained a U.S. patent (U.S. Pat. No. 5,240,792) on a network type solid polymer electrolyte in which a monofunctional acryloyl PEO monomer and a difunctional acryloyl PEO monomer are crosslinked. These crosslinked solid polymer electrolytes are mostly cured by high energy beams such as electron beams and ultraviolet lights. After the curing, self-supportive films can be obtained, but the elongation and bending properties are poor, and therefore, they are brittle. Consequently, the battery manufacturing process using these polymer electrolytes becomes very complicated if they are not directly coated on the electrode. Meanwhile, Korean Patent Application No. 97-66096 discloses a solid polymer electrolyte in which the poly(ethylene glycol) dimethacrylate as crosslinking agent has an increased number of the PEO repeating units to partly reinforce the mechanical properties thereof.
A feature of the present invention is the provision of a new crosslinking agent for manufacturing a solid polymer electrolyte, in which the mechanical properties and the ion conductivity are improved.
Another feature of the present invention is the provision of a solid polymer electrolyte having a three dimensional network structure which is formed by introducing three ethylene glycol acrylate molecules into a cyclic alkyl or hetero-alkyl molecule.
A further feature of the present invention is the provision of a solid polymer electrolyte having a hard segment comprising a 6-membered cyclic structure to which three linear or branched polymer chains are bonded at equal distances from one another, and a soft segment formed by the three linear or branched polymer chains of an oligomeric poly(ethylene oxide). The net structure formed by the hard segment and soft segment functions to improve the mechanical properties of the solid polymer electrolyte and the oligomeric polyethylene oxide functions to dissolve the lithium salt. The resulting solid polymer electrolyte is suitable for use in a lithium polymer secondary battery.
In accordance with one aspect of the present invention, there is provided crosslinking agent including a compound represented by formula (I): 
wherein
A is oxygen, COO, alkyl of C1-4, or a single bond,
R is a 6-membered aliphatic, aromatic or heterocyclic group,
Ra, Rb and Rc independently are a linear or branched alkyl group of C1-10,
Rd, Re and Rf independently are H or a methyl group, and
p, q and r independently are an integer from 1 to 20.
In more specific embodiments, the inventive crosslinking agent includes a group R which is selected from the group consisting of cyclohexane, benzene, triazine, trioxane and isocyanurate.
In accordance with another aspect of the present invention, there is provided a solid polymer electrolyte including a crosslinking agent as described above; a poly(alkylene glycol) alkyl ether alkyl (meth)acrylate of formula (II) 
wherein
R1 and R2 independently are a linear or branched aliphatic or aromatic group of C1-10,
R3, X, Y, Z independently are H or a methyl group, and
p, q and r independently are an integer from 1 to 20; a lithium salt; and a curing initiator.
According to more particular embodiments, the inventive solid polymer electrolyte further includes a poly(alkylene glycol) dialkyl ether of formula (III): 
wherein
R1 and R2 independently are a linear or branched alkyl group of C1-10,
X, Y and Z independently are H or a methyl group, and
p, q and r independently are 0 or an integer from 1 to 20.
In accordance with still another aspect of the present invention, a lithium polymer secondary battery including an anode, a cathode, and a solid polymer electrolyte as described above is provided.
According to an additional aspect of the present invention, there is provided a solid polymer electrolyte including: a crosslinking agent having a hard segment comprising a 6-member cyclic structure, and soft segments including three linear or branched oligomeric poly(ethylene oxide) chains substituted on the hard segment at the 1,3 and 5 positions; a poly(alkylene glycol) alkyl ether alkyl (meth)acrylate of formula (II) above; a lithium salt; and a curing initiator.
Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.